The use of amorphous hydrated metal oxide and amorphous metal silicate gels to remove various metals from aqueous solutions is notoriously old in the art. The patent and technical literature contains many examples of such techniques. It was recently discovered that certain amorphous titanium and tin silicate gels demonstrated remarkable rates of uptake for heavy metal species such as lead, cadmium, zinc, chromium and mercury which are an order of magnitude greater than that of prior art absorbents or ion exchangers under the conditions tested which include the presence of competing ions such as calcium and magnesium. The combination of extraordinary lead selectivity, capacity and uptake rates, permitted such materials to strip lead from aqueous streams with minimal contact time, allowing direct end use in filters for water purification, be it under-the-counter or under-the-faucet, or whole-house devices. See U.S. Pat. No. 5,053,139.
Granular activated carbon (GAC) is also used in water purification or remediation processes, in some cases in combination with inorganic metal ion-exchangers. GAC improves taste, odor and smell by adsorbing ionic metals, organic molecules and colloidal particles. GAC also removes chlorine by reducing the chlorine to chloride ions.
The adsorption properties of GAC are dependent upon the pore volume and pore size distribution of the material. Small organic materials., e.g., volatile organic compounds (VOC) such as chloroform, are adsorbed into the micropores (pores less than 20 angstroms in diameter). Larger organic materials, e.g., herbicides or pesticides or colloidal particles are adsorbed in the mesopores (pores between 20 and 500 angstroms). The macropores (pores between 500-2K angstroms) are least important for adsorption, but do trap larger sediment particles.
Prior art directed to composites of support particles such as activated carbon and inorganic ion-exchangers include U.S. Pat. No. 4,692,431, Weller and U.S. Pat. No. 4,178,270, Fujita, et al. These patents deal with the problem of providing mechanically strong particles of inorganic ion-exchangers and do so by precipitating metal oxide gels on support materials. With efforts directed to securing an effective bond between these components, patentees did not attempt to utilize significantly the inherent benefit of the porous support itself to bind material not captured by the inorganic ion-exchanger. Thus, Weller intentionally deposited precipitated metal oxide within pores of a carbon support. Fujita, et al. relied on utilizing a support and precipitated metal oxide having opposite zeta potential while ignoring the possibility of retaining porosity in the support.